The present invention relates to a method and an apparatus for preparing polysilicon (or polycrystalline silicon), more specifically to a method and an apparatus for preparing polysilicon in granule form by equipping a fluidized bed reactor with a nozzle that provides an etching gas including hydrogen chloride in order to effectively prevent silicon from depositing on the outlet surfaces of the reaction gas supplying means and to be able to operate the reactor continuously in the bulk production of polysilicon granules.
High-purity polysilicon is used as a raw material of semiconductor-grade single crystal or solar cell-grade silicon substrate for photovoltaic application. The polysilicon is prepared by the chemical vapor deposition method that deposits silicon continuously on silicon surfaces through thermal decomposition or hydrogen reduction of silicon-containing gas.
For the commercial bulk production of polysilicon, a bell-jar type reactor is generally used. The polysilicon prepared with this reactor has rodlike form with a diameter of about 50-300 mm. Since a bell-jar type reactor, which requires electrical resistance heating, is limited in its rod diameter, it cannot produce products continuously and a large amount of power is consumed to keep the temperature of the silicon rod surfaces above 1,000xc2x0 C.
Recently, a silicon deposition process using a fluidized-bed type reactor, which produces polysilicon in granule form with a particle diameter of about 0.5-5 mm, has been proposed in order to solve said problems. According to this process, a fluidized bed of moving silicon particles is formed by the reaction gas supplied from the lower part of the reactor toward its upper part. Elementary silicon is continuously deposited on the hot surfaces of the fluidizing silicon particles, which grow into polysilicon product granules. Being enlarged from the smaller seed crystals due to the repeated silicon deposition, the larger particles tend to lose mobility and to settle downward. Here, the seed crystals can be supplied continuously or periodically into the fluidized bed, and the enlarged particles can be withdrawn continuously or periodically from the lower part of the reactor.
The polysilicon prepared using a bell-jar type reactor or a fluidized-bed reactor is substantially used for the preparation of silicon single crystal, which is a fundamental material of semiconductor wafer. The silicon single crystal is produced mostly with a Czochralski-type grower, where the high-purity polysilicon feedstock is heated to its melting point of about 1,400xc2x0 C. in a crucible and then a single crystal is slowly grown up from the silicon melt. In the crystal growing process, while the polysilicon granules can be charged directly into the crucible of the crystal grower, the rodlike polysilicon produced in a conventional bell-jar type reactor should be subjected to crushing and sorting processes before being charged into the crucible. Also, complicated processes like etching with a high-purity inorganic acid, washing with ultra pure water, drying and packaging under clean atmosphere are additionally required to remove the impurities of the silicon surfaces that are contaminated during said crushing and sorting processes.
Because the polysilicon in a rod form produced with a bell-jar type reactor has such disadvantages as serious material loss during the additional treatment processes and increased costs related with the removal of impurities, the polysilicon granules produced by a fluidized bed reactor are expected to gradually replace the rodlike product in the future.
Another advantage of the fluidized bed reactor is that a much higher reaction yield can be obtained in case of the silicon deposition of silicon particles with very large surface area compared with that on silicon rods in the bell-jar type reactor under the same reaction condition.
Since the silicon-containing gas begins decomposition at the temperature higher than 300-400xc2x0 C., an initial decomposition temperature, the reaction for silicon deposition can proceed on any solid surfaces within a fluidized bed reactor if the reaction temperature is higher than the initial decomposition temperature. Silicon can be deposited on the hot surfaces irrespective of their types and material composition. Therefore, silicon deposition and its accumulation may occur not only on the surface area of the fluidizing silicon particles but also on that of reaction gas supplying means whose temperature is maintained nearly as high as the reaction temperature.
The problem of accumulation of silicon deposit on the surfaces of the reaction gas supplying means is the severest at its outlet side, where the reaction gas is injected into the fluidized bed. If silicon is deposited on the outlet surfaces, which are in continuous contact with high-temperature silicon particles, the temperature of the outlet surfaces is maintained to be nearly the same as that of those particles. Silicon deposition by the injected reaction gas thus always proceeds also on the outlet surfaces of said reaction gas supplying means, by which the thickness of the accumulated silicon deposit at the outlet surfaces should continue to increase gradually.
This unwanted accumulation of silicon deposit interferes with the continuous operation of the fluidized bed reactor, which is a serious problem for bulk production of polysilicon granules. The degree of accumulation of the silicon deposit depends, somewhat differently, on the geometry of the outlet of the reaction gas supplying means, the shape of reaction gas injection and the pattern of contact between the fluidizing silicon particles and the accumulated silicon deposit. But its accumulated amount increases with operation time to change the geometry of the reaction gas outlet and ultimately to cause clogging.
In addition to these problems, the accumulation of silicon deposit may induce the physical or thermal deformation and stress due to the deposited layer or lump and cause a crack or damage of the reaction gas supplying means itself.
In order to solve these problems, a cooling medium such as cooling water, oil or gas may be circulated into the reaction gas supplying means to keep its surface temperature below a predetermined value. Otherwise, preheating of the reaction gas outside the reactor needs to be minimized to lower the temperature of the reaction gas supplying means. In this case, the cooling of the reaction gas supplying means itself should be thorough in order to decrease the surface temperature of the reaction gas supplying means exposed to the inside of the reactor at about 1,000xc2x0 C., especially the surface temperature of the reaction gas outlet, below the initial decomposition temperature of the silicon-containing silane gas. However, because a high-purity inorganic material with low thermal conductivity should be used for the reaction gas supplying means to prevent the contamination of the reactor inside due to impurities, it is practically impossible to lower the temperature of the reaction gas supplying means sufficiently. Even being assumed to be possible, such sufficient cooling of the reaction gas supplying means will result in the quenching of the reaction gas.
As seen above, cooling of the reaction gas supplying means removes too much heat from the fluidized bed through radiation, convection and conduction. It is notable that the heating of fluidized bed within the reactor is the most important and difficult issue in the production of the granular polysilicon, especially when the production rate is high. Therefore, an intense removal of energy from the reactor inside due to thorough cooling of the reaction gas supplying means is undesirable. Moreover, the low-temperature reaction gas resulted from the cooling should lower the surface temperature of the silicon particles where the deposition reaction proceeds. This leads to reduction in production rate and reactor efficiency.
U.S. Pat. Nos. 4,150,168 and 4,786,477 disclose a method of providing silane gas with a temperature lower than the initial decomposition temperature into the reactor. In this method, a cooling medium such as cooling water is used to artificially cool the reaction gas supplying means in direct contact with the fluidized bed. Also being cooled thereby, the low-temperature reaction gas discharged from the reaction gas supplying means causes the fluidizing silicon particles to be quenched inevitably. Therefore, heating of the reactor inside becomes more difficult according to such proposed method. What is worse, the problem of accumulated silicon deposition on the surfaces of the reaction gas supplying means is not practically solved even with this method.
Accordingly, there is a serious problem of the silicon deposition on the surfaces of the reaction gas supplying means, especially at its outlet, being originated from the direct contact of the reaction gas supplying means with high-temperature silicon particles of the fluidized bed reactor. However, no appropriate solution has been provided yet because of the inherent configuration of the fluidized bed reactor.
As mentioned above, a method of producing polysilicon using a conventional bell-jar type reactor cannot produce the polysilicon continuously; power consumption is large; and post-treatment is required before use. A fluidized bed reactor can solve to some extent those problems of the bell-jar type reactor. However, continuous operation of the fluidized bed reactor is impossible unless the accumulation of silicon deposit on the surfaces of the reaction gas supplying means is prevented.
Therefore, an apparatus and a method to prevent the accumulation of silicon deposit on the surfaces of the reaction gas supplying means should be developed for continuously preparing polysilicon with a fluidized bed reactor.
Therefore, an object of the present invention is to provide an apparatus and a method for preparing polysilicon, which prevents the accumulation of silicon deposit on the surfaces of the reaction gas supplying means and enables continuous operation of the reactor. The present invention achieves the object by injecting an etching gas including hydrogen chloride on the surfaces of the reaction gas supplying means of the fluidized bed reactor.